Streptococcal M-protein has been extracted from the cell walls of Group A streptococci with nonionic detergent. M-protein is then purified by ion-exchange and Sephadex chromatography and subsequently used in chemical and immunological analyses. M-protein isolated by this method has been found to be composed of subunits which have the ability to reaggregate into trimers. The chemical composition of M-protein isolated from various M types is being compared by peptide mapping in order to see if there exist common peptides within certain types. A peptide mapping system using ion exchange chromatography has been developed in order to facilitate this type of analysis. Amino acid analysis of M-proteins isolated from "throat" strains have been found to be quite similar to those M-proteins isolated from "skin" strain streptococci. M-protein isolated from "throat" strain streptococci has been used successfully in a radioimmune assay in order to determine the presence of type-specific opsonic antibodies in human serum. Work is now in progress to correlate bactericidal data with a radioimmune assay directed against M-protein isolated from pyoderma strain streptococci.